Magnetic focusing system



April '28, 1970 R. E. JOANNAIS ETAL MAGNETIC FOCUSING SYSTEM Filed March 7, 1968 ilirlrlllllillfllilli United States Patent 3,509,504 MAGNETIC FUCUSING SYSTEM Roger E. .loannais and Raymond Defain, Paris, France, assignors to CSF-Compagnie Generale de Telegraphic Sans Fil, a corporation of France Filed Mar. 7, 1968, Ser. No. 711,448 Claims priority, application; France, Mar. 14, 1967, 98 6 4 Int. Cl. 1 1011 3/12 US. Cl. 335211 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to magnetic focusing systems of the kind used for beam concentration purposes in O-type travelling wave tubes.

It is Well known that in magnetic focusing devices, the useful magnetic field is the stronger the higher the coercive force of the magnets. For this reason, ferrites having a coercive force (H :3000 oersteds) higher than that of other magnetic materials, are generally used. Unfortunately, the magnetic properties of ferrites are highly sensitive to temperature variations. The temperature coefiicient of ferrites is in the order of 0.2% per degree centigrade and a temperature variation of 80 C. produces a field loss of about 16%. Yet, for proper operation of tubes using periodic permanent magnet devices, as in travelling wave tubes, it is essential that the focusing field should be as stable as possible.

This problem has already been overcome in a variety of ways, each, however, giving rise to a relatively complicated structure.

It is an object of this invention to provide a focusing system, which avoids such drawbacks.

According to the invention, there is provided a magnetic focusing system comprising annular ferrite pieces having polarities alternating from one piece to the next, spaced by pole pieces and forming a cylindrical sleeve; and a further cylindrical sleeve of magnetic material positioned within said first mentioned sleeve for shunting said pole pieces, said further sleeve being arranged for receiving that portion of a travelling wave tube through which a delay line extends.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawing accompanying the following description and in which:

FIG. 1 illustrates a travelling wave tube employing a magnetic focusing system according to the invention;

FIG. 2 illustrates the focusing system on an enlarged scale; and

FIG. 3 i an explanatory graph.

In FIG. 1, a travelling wave tube 1 has been illustrated, comprising an electron-gun 2 near one end and a helical delay line 3 between the gun and the other end of the tube where the collector 7 is located.

The part of the tube containing the helical line 3 is located within a magnetic focusing system which comprises a cylindrical sleeve 4 placed within another sleeve formed of annular ferrlte magnets 5, magnetized in the direction parallel to the tube axis, whose polarities are 3,509,504 Patented Apr. 28, 1970 alternately inverted so that they are NS, SN, NS, SN etc., and between which annular pole pieces 6 with a T shaped section are inserted.

FIG. 2 illustrates on an enlarged scale a part of the focusing system shown in FIG. 1.

In accordance with the invention, the sleeve 4 is made of a ferromagnetic material, for example of 25 to 35% of nickel and to 65% of iron, whose induction varies rapidly as a function of temperature. The magnetic properties of a material of this sort are illustrated in the graph of FIG. 3, in which the temperature in degrees centigrade is plotted along the abscissa and the saturation flux B in kilogauss along the ordinates.

The sleeve 4 is made of ferromagnetic material and forms a shunt for the magnets 5 and pole pieces 6.

In operation, any increase in temperature of the system results in a simultaneous reduction of the induction in the magnets 5 and in the magnetic sleeve 4. Consequently, the field produced by the magnets 5 reduces, but the shunt effect created by the sleeve reduces too, since its induction at saturation drops 0E. The proportion of the mag netic flux taken by the sleeve is therefore less than before the temperature rise.

For proper operation, the two effects should balance one another and the useful flux remain therefore substantially unchanged and the amplitudes of the alternating field along the axis of the focusing system remain substantially constant. In other words, temperature effects are compensated.

In one embodiment of the invention, a variation of less than 1% in the amplitudes of the alternating field was observed, with the temperature rising from 20 to C.

Of course the invention is not limited to the embodiment described and shown which was given solely by way of example.

What is claimed is:

1. A magnetic focusing system comprisingzannular ferrite pieces having polarities alternating from one piece to the next, pole pieces spacing said annular pieces and forming a cylindrical sleeve; and a further cylindrical sleeve of a magnetic material, whose induction is temperature dependent, positioned within said first mentioned sleeve for shunting said pole pieces for achieving a magnetic field compensating effect extending through the whole of said focusing system, said further sleeve being arranged for receiving that portion of a travelling wave tube through which a delay line extends.

2. A focusing system as claimed in claim 1, wherein the magnetic material of said further cylindrical sleeve comprises 25 to 35% of nickel and 75 to 65 of iron.

3. A focusing system as claimed in claim 1, wherein said pole pieces are T shaped, the horizontal bar of said T being positioned between said two sleeves.

4. A travelling wave tube having a focusing system as claimed in claim 1.

References Cited UNITED STATES PATENTS 2,812,469 11/1957 Kleinet a1. 315 3.5 2,843,775 7/1958 Yasuda 313--84 2,867,745 1/1959 Pierce 315 3.5 3,061,754 10/1962 Kajihara 313 s4 3,178,602 4/1965 Meixner 313 84 3,271,616 9/1966 Verger 313-84XR GEORGE HARRIS, Primary Examiner US. Cl. X.R. 315-35; 335-217 

